1. Field of the Invention
The present invention relates generally to devices for the transformation of part of the kinetic energy of a moving fluid into pressure of the fluid. More particularly, the invention concerns a diffuser for use in connection with gas turbines.
2. Discussion of the Invention
A novel and highly successful short subsonic diffuser is described in U. S. Pat. No. 4,029,430 issued to the present inventor. Another example of a novel diffuser is described in U.S. Pat. No. 3,599,431 also issued to the present inventor. Because of the pertinence of these two patents, and because the present invention comprises an improvement over the teachings of these patents, both U.S. Pat. No. 3,599,431 and 4,029,430 are incorporated herein as though fully set forth in this application.
U.S. Pat. No. 4,029,430 describes an extremely efficient diffuser which can recover kinetic energy in a gas stream and convert it to an increase of pressure in the duct. This means that the pressure at the exit of the duct is higher than at the inlet, while the gas velocity there is lower. If the exit of the duct is at atmospheric pressure, the pressure at the inlet is lower than atmospheric and becomes lower still if the gas at the inlet carries substantial kinetic energy and if the diffuser can convert this energy efficiently into a pressure increment.
It is, of course, well known that gas turbines operate by expanding a gas stream from a high pressure in the combustor down to an end pressure at the exit of the last stage of the turbine and extracting as much kinetic energy as practicable from the gas stream. However, energy extraction is never complete, and a certain fraction of energy remains in the gas, which moves with considerable speed at the exit at the last stage of the turbine, typically with a Mach number of on the order of 0.5. The diffuser attempts to recover some of this energy by increasing the pressure increment from the exit of the last stage to the final exit to the atmosphere. A more efficient diffuser generates a larger pressure increment, and since the final exit pressure is constant (the atmosphere), it lowers the pressure downstream from the last stage of the turbine. This in turn increases the pressure drop from the exit of the combustor across the stages of the turbine and consequently increases the total energy available for extraction by the turbine.
The commercial value of a highly efficient diffuser is substantial. For example, in the range of between Mach 0.5 to atmosphere, a diffuser, which is about 90 percent efficient, can increase the fuel efficiency of the turbine by three to five percent. These types of increases in fuel efficiency have been demonstrated by the physical embodiments of the inventions described in U.S. Pat. No. 4,029,430 ('430). However, the diffuser described in the aforementioned patent is not well suited for use in a gas turbine because the gas turbine typically has an inner wall as well as an outer wall on which a boundary layer develops and can become detached. Such a construction cannot readily be accommodated by the embodiments of the '430 patent. It is this drawback of the inventions of the '430 patent, along with other drawbacks of the prior art, which the present invention seeks to overcome.